Miniature inductances

ABSTRACT

Improved insulation for inductances, such as ring-shaped miniature transformers, is disclosed. An inductance is provided with a pair of similar axial end caps made of insulating material and interposed between core and winding.

United States Patent Elberger [54] MINIATURE INDUCTANCES [72] Inventor:Martin Elberger, 7020 Bothwell Road,

Reseda, Calif. 91335 Filed: Feb. 3, 1971 Appl. No.: 112,252

Related U.S. Application Data Continuation-in-part of Ser. No. 810,7l8,March 26, I969, abandoned.

{52] U.S.Cl ..336/221,336/l00,336/208, 336/210 Int. Cl. ..H01f17/04Field of Search .336/198, 208, 2 I0, 22 l 2l 3,

[4 1 July 25, 1972 Primary Examiner-E. A. Goldberg Attorney-Smyth,Roston & Pavitt 5 7] ABSTRACT lmproved insulation for inductances, suchas ring-shaped miniature transformers, is disclosed. An inductance isprovided with a pair of similar axial end caps made of insulatingmaterial and interposed between core and winding.

3 Claims, 6 Drawing Figures MINIATURE INDUCTANCES This is acontinuation-in-part application of my application Ser. No. 810,718filed Mar. 26, I969, now abandoned.

The present invention relates to improvements in the construction ofinductances, such as miniature transformers, or the like. The backgroundof the invention having lead to the improvements disclosed herein shallbe discussed first. Large scale digital memory systems usually employmemory cores arranged in a matrix pattern and accessed through an XYaddressing wire system, whereby coincidence of currents in one directionin each of a pair of such XY wires traversing the same core switches themagnetic state of the core for writing a particular bit into the coreunless such switching is inhibited otherwise. For reading the content ofthe memory core, current is driven through the same two wires butflowing in the opposite directions.

Addressing of a core, per se, by selective operation of a particularpair of such wires within the XY system is an operation which isfunctionally independent from the purpose of the addressing (the purposebeing either writing or reading). The direction of current flow definesthe purpose. Thus, it is necessary, within the chosen wiring diagrams,to isolate voltage potentials as provided by the addressing logic,including some or all of the decoding circuit, and providing signals foraddressing a core through a pair of XY wires, from the sources drivingswitching currents through the XY wires.

Therefore, it is common in digital memories to interpose transformers,particularly miniature type pulse transformers, between the circuitryreceiving addressing signals and decoding same on one hand, and the XYdrive system on the other hand. The transformers can be small becausevoltages and currents transmitted are quite small. Moreover, thetransformers should be small because they must fit into the overallmodular construction pattern usually employed, according to which allcircuit elements are mounted on relatively small printed circuit boards.

Logic elements, amplifiers, and other signal processing circuitry usedtoday is solid state. This includes particularly the decoding andamplifying circuit of a memory system, as

described. The transformers within that system are, therefore, from astandpoint of modern development, the oldest" type circuit elementsemployed. Surprisingly, it is that element which has proven to be mosttroublesome within the system. The transformers employed comprise asmall ring-shaped core with thin insulated wires wound thereupon in afew turns. It was found that the core abrades the insulation from thewindings and the baredwire is short circuited to the core.

Often such short circuit can be detected immediately upon testing thetransformer, prior to installation because the abrasion occurs primarilyduring the wire winding process. However, it was found also that in manycases there is some abrasion initially, but insufficient to establishimmediately metalto-metal contact between partially abraded wire andcore. Even though the signal frequencies transmitted by the transformerare very high, there is still some mechanical interaction between wiresand core. More importantly, however, there is thermal expansion. Suchmechanical action causes additional abrasive action between core andwires during operation, which, in turn, causes the insulation to bescraped off, particularly in those places where there was already someabrasion during the winding process.

It was found that actually such seemingly minor damage could causeoperational breakdown of an entire computer system. There is, therefore,a definite need for improvement in the reliability of such miniaturehigh-frequency, high speed pulse transformers.

The obvious solution to the problem of avoiding core-towire contactwhere wire insulation has been abraded is to provide the core with aninsulating coating. This has been tried; however, such coating posesmore problems than it solves. It was found, for example, that an epoxycoating, or the like, after hardening constricted the ferrite cores usedfor pulse transformers, as described. As ferrite is a magnetostrictivematerial, the magnetic properties of the core were materially altered.Inductance changes up to 25 percent have been observed. Other coatingmaterial was tried, but the thermal curing process required proved alsodetrimental to the magnetic properties of the core.

Still other coatings were found to form thick layering so that thecentral aperture of the ring-shaped core became too small for threadingthrough the required number of turns, at least in a sufficiently fastoperation. To accommodate a thicker coating, it would be required tomake the cores somewhat larger, but the transformer should be made assmall as possible so that any constraint requiring increase in size butnot having to do with the operation proper should be avoided.

Vapor deposition of an insulating material on the core has been triedalso, but it was found that sharp metallic gratings on the core areinsufficiently covered therewith, producing both abrasion of wireinsulation and metal-to-metal contact with the bared wire thereafter.

In accordance with the invention, a solution to the problem, posed bythese transformers, was found. The improvement in accordance with thepresent invention, however, will find utility for other core-coilconstructions as well. In accordance with the invention, it is suggestedto make individual end caps of insulating material and to place theinsulating end caps on a ring core, prior to winding wires thereon sothat the end caps provide insulative spacing between the subsequentlyplaced wires and the core. There are two end caps for each core, eachend cap having a central hub preferably for frictional engagement withthe surface of the ring core defining the central aperture thereof. Eachend cap forms a smooth surface side wall gripping around the peripheryof the core and extending along the cylindrical circumference thereof toserve as a spacer for the wires. The end cap yields resiliently to thetensioning force to establish a gently curving configuration matchingthe contour of the wire loops. Some resilient reaction tensions thewire, but quite gently, so that there is no loose fit. The frictionalengagement of end cap portions by the core must be limited to just suchengagement without exerting significant constricting forces upon thecore, when made of ferrite, so as to avoid magnetostrictive actiontherein.

While the specification concludes this claim, particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention, and further objects, features and advantages thereof,will be better understood from the following, taken in connection withthe accompanying drawings, in which:

FIG. 1 illustrates a radial section view through a ring-shapedtransformer provided with end caps in accordance with the presentinvention;

FIG. 2 illustrates a perspective view of the end cap as used in thetransformer shown in FIG. 1;

FIG. 3 illustrates a perspective view of another end cap;

FIG. 4 illustrates a radial section view through a transformer coreprovided with an end cap, as shown in FIG. 3, prior to winding wire ontothe core;

FIG. 5 illustrates the core with end caps, as shown in FIG. 4, but afterwinding wires thereon; and

FIG. 6 illustrates a radial section view through still another end capin accordance with the invention.

A coil core structure, such as a miniature transformer, improved inaccordance with the first embodiment of the present invention isillustrated in FIG. I. The transformer includes an essentiallyring-shaped or toroidal core 10, illustrated in the Figure in aconsiderably enlarged scale. The outer diameter of such a transformercore is usually less than one fourth of an inch, if the transformerserves as pulse transmitter in core memory drive circuits. Core 10, whenserving as a transformer core, carries windings, denoted generally aswindings I1, and including primary and secondary windings in aconventional manner and wound upon the core usually manually. The wires,however, are not wound directly on the core.

In accordance with the feature of the present invention there areprovided two end caps such as end caps 12 and 12'.

These two end caps are similar and are thus disposed symmetrically to anaxial central plane through ring core 10. The caps are termed end capsas they abut oppositely oriented axial end faces of the cylindrical ringcore 10. Cap 12 is illustrated separately in FIG. 2 and in perspectiveview.

The end cap has a flat, ring-shaped portion, or annulus 20 which abutsone of the axial faces of core when the core is placed thereon, as shownin FIG. 1. The outer diameter of annulus is larger than the outerdiameter of the core. End cap 12 has, in addition, an annular wall 21.After placement of the end cap on ring-shaped core 10, wall 21 extendsaxially parallel in relation to the center axis of the core. Thus, thewall 21 grips around the outer circumference of the core.

End cap 12 has, furthermore, tubular portion 22 extending from thecentral aperture of annulus 20 and resembling a hub. Outer wall 21, hub22 and bottom annulus 20 define a ring or annular trough, and one cansay that upon placing the cap on the core, the core is seated in thetrough but projects therefrom. Core 10, in particular, is snuglyreceived in that trough and, therefore, positively positioned therein.

There are provided, as shown in FIG. 1 and as already mentioned above,two of these end caps, 12 and 12', for each core. The end caps aresimilar, i.e., the same type of caps are used for each of the two axialend faces of a core. There is no principle necessity for such similaritybut it is practical to use one type of cap. It appears, therefore, thatthe wires 11 are wound in reality on these end caps. The two end capsthus serve as spacers for the wires.

The end caps could be constructed such that the two axial extending sidewalls 21 and 21' abut with the axial surfaces, but this would requireunnecessary high precision. Therefore, in order to ensure snug seatingof each of the two end caps on core 10, it is preferred to leave a gap,such as gap 13, inbetween the two axially aligned side walls 21 and 21'.Analogously, there is a gap 14 between the two axially aligned hubs 22and 22'. The dimensions are chosen such that wires 1 1 will span the twogaps at a sufficient distance from the core.

It can thus be seen that nowhere can the wires engage core materialdirectly so that even if the insulation of the wires is abraded to someextent, during the winding process and/or afterwards, there will be nometal-to-metal contact with the core. Moreover, the plastic material forthe end caps can be chosen such they have rather soft surfaces so thatthere is little or no abrasion during winding of the wires onto the endcaps, nor will there be significant abrasion subsequently under theinfluence of electrical and thermal forces when wires may tend tovibrate physically or to move otherwise relative to the structure towhich they are mounted. This way insulation, usually lacquer, on thewires will not, or will hardly, be damaged. The end cap illustrated canpreferably be made of nylon which has been proven to satisfy therequirements.

Since miniature transformers of the type envisioned here are usuallywound manually on the core, there is practically no additional workinvolved for positioning the end caps on the core prior to winding wiresthereon. The dimensions of the end caps should be chosen so that theyfit snugly over the core, i.e., in press-fit. The ring trough, asdefined by ring 20, wall 21 and hub 22, should have an inner ring widthslightly smaller than the ring width of the core to frictionally receivethe core such that, once end caps are placed on the core, they will notfall off by themselves, but will require additional force to be removed,which force, of course, is normally not applied. This is important as itpermits handling of the cores freely during the wire winding process;thus, after the end caps have been placed onto the core the wires can bewound thereon in a conventional manner.

The end caps shown in FIG. I and FIG. 2 are comparatively expensive astheir manufacturing requires some lathing operation. FIGS. 3, 4 and 5illustrate a simpler configuration for such end caps. The configurationis chosen such that they can be made through injection molding. In thisparticular embodiment there is a flat ring 30 which is rather thin andpliable. Outer diameter of the ring is again larger than the diameter ofthe core upon which the cap is to be placed. A hub-like ring 31 extendsaxially from the center of ring-shaped disc 30.

As illustrated in FIG. 4, these thin end caps are slipped onto a core 10with the hub portion 31 of a cap being inserted in the central apertureof a ring-shaped core 10. Again, the dimensions of the hub should bechosen wide enough so that there is press-fit through frictionalengagement between hub and core. The thin disc 30 projects beyond theouter diameter of the core 10. As wire is wound around the core,preferably under tension, the projected portions of the ring disc 30 arebent axially as they yield resiliently. The resulting smoothly curved,axially bent portion serves as resilient spacers for the wires. Someresilient reaction of the bent portion of the disc tensions the wire andis thus instrumental in maintaining the wire in position. Even as thewire is wound on the core and the edge of disc 30 is yieldingly bent,there is little danger of abrasion, due to smooth curving of the softsurface disc as bent. The bending is not regular around the periphery ofthe core, but bending occurs only where the wire engages edge 32 of ring30. Nevertheless, as a result of winding the wires over the core, a sidewall is established by bending the disc axially, on the periphery.

Nylon was found to be the most suitable material for such type of endcaps, and, as stated, these end caps can be made by injection molding ofnylon. For this reason they are actually more economical than the endcaps shown in FIGS. 1 and 2, but they still suffice for the desiredpurpose. They provide adequate spacing for the wires for separating themfrom the core to prevent abrasive action and to prevent contact with themetal of the core if abrasion did occur. As stated, the cap is retainedon the core by press-fit of the hub, particularly prior to and duringwinding of wire thereon. Of course, once the wire is wound upon thestructure, the wire serves additionally to retain the end caps on thecore.

FIG. 6 illustrates a still further embodiment for an end cap which canbe made also of nylon but can possibly be made also of a polycarbonatecalled Lexan. End cap 40, illustrated in this Figure, has a hub 41 and abeveled wall 42. An end cap having such configuration can be lessresilient as the outer circumference does not have to be bent over thecore by the wire. Lexan is less resilient than nylon, but Lexan issuitable particularly because such an end cap can be made by hotstamping. Alternatively, of course, nylon could be used with injectionmolding being the most suitable way of preparing such end caps.

The invention is not limited to the embodiments described above but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention ar intended to be included.

Iclaim: f

l. A miniature inductance element having ring-shaped core meansextending around an axis for magnetic interaction with one or moreelectrical windings, a pair of similarly shaped end caps made ofsoft-surface, resilient and yielding plastic and being individuallymounted on the core means in symmetrical relation to each other and withreference to an axial plane running through the center of the coremeans, the windings being wound around and disposed on smooth surfaceportions of the caps as placed onto the core means, the caps each havinga hub extending into the central aperture of the ring of the core meansand axially facing the hub of the respective other caps, each capfurther having a flat annulus from which the respective hub extends,each annulus having a bevelled peripheral portion extending adjacent aportion of the outer periphery of the core toward the bevelled portionof the respective other annulus to maintain the windings in spaced apartrelationship to the core means, the bevelled portions of the end caps aswell as the hubs spaced apart from each other.

2. A miniature inductance element having ring-shaped core meansextending around an axis for magnetic interaction with one or morewindings, a pair of ring-shaped discs each disc having a centralaperture and disposed on the core means, coaxial thereto each dischaving a hub extending axially from the respective central aperture ofeach disc, the discs further disposed symmetrical to a plane normal tothe axis and running through the center of the core means, the innerdiameter of each the central aperture of the discs being smaller thanthe inner diameter of the central aperture of the ring of the coremeans, the outer diameter of each of the discs being larger than theouter diameter of the ring of the core means, each of the discsconstructed from soft-surface resiliently yielding plastic material;

electrical windings wound about the discs, the windings

1. A miniature inductance element having ring-shaped core meansextending around an axis for magnetic interaction with one or moreelectrical windings, a pair of similarly shaped end caps made ofsoft-surface, resilient and yielding plastic and being individuallymounted on the core means in symmetrical relation to each other and withreference to an axial plane running through the center of the coremeans, the windings being wound around and disposed on smooth surfaceportions of the caps as placed onto the core means, the caps each havinga hub extending into the central aperture of the ring of the core meansand axially facing the hub of the respective other caps, each capfurther having a flat annulus from which the respective hub extends,each annulus having a bevelled peripheral portion extending adjacent aportion of the outer periphery of the core toward the bevelled portionof the respective other annulus to maintain the windings in spaced apartrelationship to the core means, the bevelled portions of the end caps aswell as the hubs spaced apart from each other.
 2. A miniature inductanceelement having ring-shaped core means extending around an axis formagnetic interaction with one or more windings, a pair of ring-shapeddiscs each disc having a central aperture and disposed on the coremeans, coaxial thereto each disc having a hub extending axially from therespective central aperture of each disc, the discs further disposedsymmetrical to a plane normal to the axis and running through the centerof the core means, the inner diameter of each the central aperture ofthe discs being smaller than the inner diameter of the central apertureof the ring of the core means, the outer diameter of each of the discsbeing larger than the outer diameter of the ring of the core means, eachof the discs constructed from soft-surface resiliently yielding plasticmaterial; electrical windings wound about the discs, the windingsbending and holding the peripheral portions of the discs axially alongthe outer periphery of the core means, the peripheral portions of thediscs tensioning the electrical windings, whereby the windings are heldin a tight fit position about the discs.
 3. The miniature inductiveelement as set forth in Claim 1, the outer diameter of the hub beingslightly larger than the diameter of the center opening of the coremeans to be frictionally retained therein.